Magnetic substance having sharp permeability transition temperature, process for making, and apparatus

ABSTRACT

A magnetic mateial which includes an impurity metal of relatively large atomic diameter such as zinc, titanium, zirconium, yttrium, or cadmium introduced uniformly therein in amount sufficient to distort its crystaline structure to alter its anisotrophy so that the permeability of the substance remains substantially constant with changes in temperature up to the permeability transition temperature and; there being also introduced uniformly therein further metallic atoms which achieve a desired transition temperature within an accuracy of at least 0.1° C. such material being utilized in an apparatus adapted to be interrogated to disclose whether its temperature is above or below the transition temperature within at least 0.1° C., the signal propagating properties of the material altering drastically at such temperature, the material having an output which is inherently digital and directly compatible with digital information processing and control without A/D conversion.

This is a continuation of application Ser. No. 684,870, filed Dec. 21,1984, now abandoned, which is a division of application Ser. No.161,689, filed June 23, 1980, now U.S. Pat. No. 4,490,268 of Dec. 25,1984, which is a division of application Ser. No. 881,592 filed Feb. 27,1978, now U.S. Pat. No. 4,208,911, of June 24, 1980, which is acontinuation of application Ser. No. 781,952 filed Mar. 28, 1977, nowabandoned, which is a continuation of application Ser. No. 293,596,filed Sept. 29, 1972 now abandoned.

The invention involves a magnetic structure which abruptly changes atits critical temperature (Curie temperature) from a magnetic to anon-magnetic state and which can be interrogated and its statedetermined by approximate external circuitry. More particularly, theinvention pertains to the method of achieving the desired abrupt changeat a preselected critical temperature and the utilization of thisphenomenon in a non-saturated linear signal propagating apparatuswherein the signal propagating properties of the material are alteredsharply after a critical temperature change of less than 0.1° C.

Magnetic materials such as spinel ferrites are well known in the art andhave a physical characteristic of a critical temperature known as theCurie temperature at which they lose their magnetic properties. Also, itis generally well known that some properties such as magnetizationchange gradually with temperature changes whereas other characteristicssuch as the initial susceptibility may disappear abruptly at thecritical temperature. With magnetic devices wherein the initialsusceptibility (or permeability) of the magnetic material is important,their performance is generally required to be independent of temperatureand; therefore they operate at a temperature well below the criticaltemperature. Such devices include inductors, transformers and the like.

It is not uncommon to measure or to utilize the change in magnetizationwhich changes with temperature to determine the induced voltage of agenerator or temperature detector or the like and to control the output.However, such mechanisms have difficulty in that the magnetization has aslow rate of change with temperature at any given point thus creating alarge area of uncertainty as to the accuracy of the temperature reading.Thus, such devices inherently have a low sensitivity to temperaturechanges.

It is also generally known that the critical temperature of a magneticmaterial is determined by its chemical and crystallographic composition.However, for many magnetic materials, this temperature varies over arelatively broad range. The difficulties associated with the preparationof a material of precisely known composition has heretofore limitedcontrol of the critical temperature to a range of several degrees. Thus,before the instant invention it has not been possible to produce amagnetic substance having a predetermined critical temperature exceptwithin a relatively broad range of say plus or minus 2° C.

SUMMARY OF THE INVENTION

The instant invention is a new method to achieve a preselectedtransition temperature within an accuracy of 0.1° C. or better throughmodification of the chemical and crystallographic composition of knowncompositions for magnetic materials, particularly spinel ferrites. Ithas thus been discovered that through the careful control of thechemical composition and thermal treatment of a modified spinel ferritecomposition the initial permeability of same may be made independent oftemperature up to a critical point where, at such temperature, thepermeability changes abruptly and drastically. In this connection, theinitial permeability of most magnetic materials is proportional to theradio of magnetostatic and anisotropic energies. Although very littlecontrol is possible over the magnetostatic energy, the anisotropicenergy may be altered by changing the impurity and imperfection contentof the magnetic material. It has been discovered that by careful controlof said impurities and imperfections, the anisotropic energy may be madeto have the same dependence on temperature as the magnetostatic energy.In this way, the ratios of said energies and therefore the initialpermeability remains independent of the temperature until the criticalpoint is reached whereupon the material becomes essentiallynon-magnetic. In the instant invention, the temperature behavior of thepermeability of a magnetic material such as a spinel ferrite iscontrolled by the adding an impurity to such a substance in an amountsufficient to distort its crystalline lattice and thus increase theanisotropy of the crystalline matrix of a spinel ferrite whereby itspermeability becomes substantially constant relative to changes intemperature up to the transition temperature. Although it is notpractical at present to produce a mixture which will result in the exactdesired transition temperature, it is possible to do so within a fewdegrees and has been found that batches from such a mixture willthereafter, as long as that particular mixture is utilized for theproduction of magnetic material in accordance with the invention, give aconstant known transition temperature.

To obtain the desired accuracy, a further modification to the chemicaland crystallographic structure of the modified spinel ferrite isrequired. It has been discovered that this can be obtained through adoping process comprising a fluid diffusion of a predetermined amount oftransition metal ions into the modified composition. In this connection,it has been observed that many metallic compounds may be prepared insolutions of desired small concentration. By using such a solution towet the starting material powder of a given composition, the solvent maybe subsequently evaporated and a uniform distribution of metallicmolecules results in the starting material. By thereafter heating thepreparation thus produced to facilitate solid state diffusion andformation of the desired magnetic compound, the organometallic moleculesdecompose and the metallic ions remain in the solid compound--theorganic gases diffusing into the atmosphere. Thus, a process is providedwhere the chemical composition of the magnetic compound is altered asdesired by varying the strength of the solution for the liquid doping ofthe starting materials with ions to attain the desired criticaltemperature. By appropriately diluting such solution of metalliccompounds with solvent, any desired degree of concentration may beobtained and a very sensitive method is provided selectively to changethe composition of the magnetic materials to within a few parts permillion. This discovery has made it possible to control the criticaltemperature of magnetic materials to within better than 0.1° C.

The general formula for a spinel ferrite in accordance with theinvention is R.sub.(1-x) T_(x) Fe₂ O₄. "R" is a metal of the iron group,atomic number 22-30, the palladium group, atomic numbers 40-48, theplatinum group, atomic numbers 72-80, or lithium. "T" is a non-magneticmetal of a group which includes zinc, titanium, zirconium, cadmium,yttrium or any R element. In order to provide temperature independenceup to the critical temperature of the substance, a further amount of "T"is added whereby the chemical formula for the substance becomesR_(1-x))T_(x/a) FE₂ O₄ where "a" is a number from 0.05 to 0.2 asnecessary to provide a substantially constant permeability of thematerial relative to temperature up to the transition temperatures. "T",although preferably a single metal from the group set forth above isused, also may be a mixture of said metals as desired.

When a further metal is diffused in the mixture set forth above toobtain the exact temperature, the formula becomes R.sub.(-x-b) T_(x-a)Fe₂ O₄ where "b" is a number in the range 10⁻¹ to 10⁻⁶ and R may beeither a single metal or a mixture of metals in the R group as set forthabove as determined to give a transition temperature which is desired.

With the transition temperature being accurately controlled by thecomposition and structure of the magnetic material, the initialpermeability of such material may be employed in signal propagatingstructure or link in communication between a signal producing apparatusand responsive means with the propagating properties of the structure orlink being drastically and discontinuously altered at the criticaltemperature. Thus, a temperature measuring device is provided havingextremely high stability and accuracy which utilizes the abrupt changesin the magnetic properties of the magnetic substance at the criticaltemperature. This may be obtained through the coils of a transformerwhich utilizes the magnetic substance as a core. With such apparatus, achange in temperature at the critical point alters the permeability ofthe material and therefore changes the mutual inductance and/or couplingcoefficient between the coils of the transformer. In a preferredembodiment of the invention, the signal propagating structure is atoroidal transformer with the magnetic material of the invention used asa core for both the primary and secondary coils. Preferably the inputsignal of the primary coil is a current ramp having a constant slope andmaximum value not to exceed the saturation drive of the magneticmaterial. Due to the natural differentiating properties of magneticinduction, the induced output voltage of the secondary coil is a squarewave of constant amplitude. A change in temperature from below to abovethe critical transition temperature abruptly changes the amplitude ofthe output voltage to near zero which is detected and responded to byelectronic circuitry external to such structure.

Other adaptabilities and capabilities of the invention will be apparentto those skilled in the art from the following description taken inconjunction with the following drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of critical temperature as a function of compositionfor nickel-zinc ferrite;

FIG. 2 is a graph showing the normalized initial permeability versustemperature curves for nickel zinc ferrite as a function of composition;

FIG. 3 is a representation of the influence of nickel doping on thenormalized initial permeability (μ_(i) /μ_(io)) versus temperaturecurves for the nickel zinc ferrite in accordance with the invention;

FIG. 4 shows the linear relationship between transition temperature andthe change in concentration of the nickel in the ferrite as introducedby the solution used for liquid nickel ion doping;

FIG. 5 is a graph illustrating the abrupt permeability versustemperature curves for manganese germanium magnetic materials;

FIG. 6 is a schematic diagram of a magnetic on-off switch whichillustrates a use of the invention;

FIG. 7 is a further schematic diagram which illustrates another use ofthe invention in the form of a digital magnetic temperature transducer;and

FIGS. 8A and 8B are a diagrammatic representation of forms of drivecurrent and output voltage which may be advantageously utilized for theapparatus shown in FIG. 7.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The practical application of the invention comprises a signalpropagating apparatus which contains a magnetic structure having thecomposition of the invention for the transmission of a time-varyingsignal such as, for example, a square wave signal. If the frequency ofthe signal is below 100 MHz, the apparatus may be constructed ofdiscrete elements with the magnetic structure being of the closed-fluxtype inductors or transformers. However, at higher frequency, theapparatus is of wave guide construction with magnetic material, inaccordance with the invention, incorporated in the proper geometry. Thematerial which, in accordance with the invention, is utilized for themagnetic structure has a critical temperature dependent upon itschemical and crystallographic composition.

As an example of the invention, FIG. 1 shows the dependence of thecritical temperature of a spinel ferrite, Ni_(1-x) Zn_(x) Fe₂ O₄, on itschemical composition. It is to be noted from the graph that thedependence is linear and that the critical temperature increases withincreased nickel concentration. In this example, the starting materialswere NiO,Fe₂ O₃ and ZnO powders of the correct proportions to obtain thedesired composition. After intimate mixing of the starting materials,they were pressed in the desired form and sintered at 1200° C. for eighthours. However, the permeability of these materials was not constant attemperatures below their transitions to their critical temperatures asdisclosed in the normalized initial permeability (μ_(i) /μ_(io)) versustemperature curves for different compositions of zinc nickel ferriteswith transtition temperatures roughly 10° C. apart.

The anisotropic energy of magnetic materials is, in accordance with theinvention, altered by the introduction of impurities or imperfectionsinto same until the temperature dependence of the anisotropic energyclosely matches the temperature dependence of the magnetostatic energy.In a preferred embodiment, impurities were introduced into the NiZnferrite by the addition of extra ZnO to the starting materials. It wasthus found that 10 percent by weight increase of the ZnO above theamount required for exact stoichiometry reduces the maximum value of theinitial permeability and makes it temperature independent up to thecritical temperature.

It was found as a practical matter, with the starting materials reagentquality pure and the weight ratios accurate to one part in a million,the critical temperature could be predetermined to about 2° C. For amore accurate determination of the critical temperature, the startingmaterials were diffused with additional metal ions by a process termedfluid "doping." It was found that doping could be advantageouslyaccomplished through the use of metal ions in liquid form such assoluble salt of the metal. The preferred doping agent was a metalliccompound of the desired metal ion such as for example for nickel doping,nickel acetate. The metal salt is dissolved in a proper solvent in thedesired concentration ratio. For the example of nickel acetate thesolvent comprises distilled water with nickel acetate dissolved thereinin a ratio of about 1 gram per 100 cubic centimeters. Other solventssuch as alcohols may be used so long as they can be eventuallyeliminated from the material. The starting material as disclosed abovehaving been intimately mixed, is wetted in such form with the solution.Thereafter with the solvent evaporated, the starting material has themetallic molecules uniformly distributed therein. The starting material,so modified, is then pressed into the desired shape for the magneticstructure and preheated at a temperature sufficient to decompose themetallic molecule into metal atoms and gases. In a particular example,the nickel acetate molecule is decomposed at 300° C. for one hour intonickel atoms, carbon dioxide and water.

Thereafter, final sintering at 1200° C. for eight hours followed by slowcooling completes the material preparation for this example. Thesintering temperature is about 50°-100° C. below the melting point ofthe ferrite involved. For the example, this is in a range of 1150°-1250°C. with the greater the amount of zinc, the lower the temperature. It isdesired to being the sintering temperature as close to the melting pointas possible without actually melting the substance. However, becauseindustrial furnaces may vary as much as from 35° C. from their settemperatures, it is advisable that the sintering be at least 50° C.below the melting point of the ferrite. The resulting material is about98-99 percent of its theoretical density.

FIG. 3 illustrates the abrupt permeability versus temperature curves forthree small toroidal cores with the transition temperatures set about 1°C. apart by the addition of nickel ions in the doping process whichincreases the critical temperature in a linear manner up to 10° C. fromthe critical temperature of the starting substance. In the Figure, theinitial permeability is normalized μ_(i) /μ_(io). The results shown inFIG. 3 were taken from three ferrite cores produced in accordance withthe invention each core being two millimeters in diameter and themeasurement was made by means of a mutual coupling between a primary andsecondary winding at 50 kHz. The linear relation between transition,temperatures and concentrations of solutions used for the nickel iondoping is shown in FIG. 4 for four ferrite samples having permeabilityversus temperature curves about 2° C. apart. The formula for suchsamples may be expressed Ni_(1-x+b) Zn_(x+a) Fe₂ O₄. where "x" is0.7455, "a' is a number from 0.05 to 0.2, (actually 0.1) as necessary toprovide a substantially constant permeability of the material relativeto the temperature up to the transition temperature and "b" is a numberin the range of 10⁻¹ to 10⁻⁶ as represented by Δ^(x) in FIG. 4. Anotherexample of magnetic materials in accordance with the invention comprisesthe composition Fe_(3-x) Mn_(x) Ge where x is a number between zero and3. This example exhibits a continuous linear change of transitiontemperature relative to x from 365° C. where x equals 0 to minus 245° C.where x equals 3. In this example, the starting materials are Fe, Mn andGe in the correct proportions to obtain the desired composition. Anotherexample is Fe_(5-x) Mn_(x) Ge₃ which has a continuous linear change oftransition temperature as a function of x between 30° C. where x equals5 and 115° C. where x equals 0. Still further compositions, Co_(1-x)Fe_(x) or Fe_(1-x) Ni_(x) exhibit a continuous change in transitiontemperature as the function of x from 1,125° C. to 355° C. In each ofthe foregoing examples, the starting materials, having been thoroughlymixed, are arc-melted in a neutral atmosphere such as argon andimmediately quenched to room temperature. This produces voids and thusimperfections to disrupt symmetry of the crystaline structure of thesubstance and to increase the anisotrophy energy. These imperfections inthe material are subsequently controlled by annealing the material at adesired temperature for the necessary length of time. In thisconnection, referring to FIG. 5, the abrupt permeability versustemperature curves for small cylindrical samples prepared from thematerial, manganese geranium (Mn₅ Ge₃) by pulverizing and pressing withan organic binder are illustrated. The curve which follows the circularrepresentations on the graph of FIG. 5 was for a cylinder prepared froma sample of Mn₅ Ge₃ which was arc-melted, quenched to room temperature,sifted through a number 230 mesh after having been powdered, andthereafter formed of particles bound with butvar. The cylindrical pelletwhich was formed by pressing had a diameter of 0.379 inches and a lengthof 0.573 inches. Its weight was 5.71 grams. The test which produced thegraph shown in FIG. 5 were performed in a solenoid two inches in lengthwith 650 turns wound on an 11 mm quartz tubing. The pellet was annealledat 900° C. for 24 hours. For the curve defined by the diamond shapedrepresentations in FIG. 5, a similar sample of Mn₅ Ge₅ was arc-melted,quenched at room temperatures and annealed in a quartz tubing at 950° C.for 24 hours. Its diameter was 0.377 and its length 0.382 inches. Forboth curves, the abrupt permeability versus temperature relationship isapparent.

In order to obtain the transition temperature with the desiredpre-selected accuracy, after melting, the material is pulverized andwetted with a solution comprising a soluble salt of the metals, iron,manganese or nickel, in the desired concentration. Thereafter, thesolvent is evaporated by heating and the metallic salt is decomposed ina reducing atmosphere such as hydrogen or carbon monoxide so that thedopant metal atoms are uniformly distributed throughout the material.Thereafter, the material is pressed into the desired shape, sintered ina neutral atmosphere, annealed at the desired temperature and thereafterslowly cooled.

In the event that eddy-current losses are important for the particularapplication, the sintered material may be again pulverized and pressedinto the desired shape with an insulating organic binder such aspolyvinyl alcohol or polyvinyl butyral.

Referring now to FIG. 6, an oscillator 10 is the source of anelectromagnetic time-varying signal. A toroidal type inductor 12composed of material in accordance with the invention is in series inthe circuit from oscillator 10 with a light bulb 14--a preferreddetection device. With the magnetic material of inductor 12 below itscritical temperature, its permeability and inductance are high and,accordingly, the impedance of the inductor is also high making thevoltage differential across the bulb 14 low which is therefore in an offcondition. With the temperature of the magnetic material exceeding thecritical temperature, the permeability, inductance and impedance are alllow whereby the necessary voltage is placed across bulb 14 which isthereby placed in a lighted or on condition. It will be appreciated thatthe toroidal inductor 12 operates as a magnetic switch without anymoving parts. It will be noted in FIG. 6 that the oscillator 10 also hasits output across a toroidal transformer 15. A light bulb 16 is inseries with the primary winding of transformer 15 and operates in amanner similar to that of bulb 14 in series with inductor 12, that is,bulb 16 is in an on condition when the temperature of the transformer isabove the critical point and is in an off position when the temperatureis below it. On the other hand, bulb 17 is in series with the secondarywinding of transformer 15. At temperatures below the critical point, thepermeability of the magnetic material in transformer 15 is high andtherefore the coupling (or mutual inductance) between the primary andsecondary winding is strong. The induced voltage in the secondarywinding is thus high and light bulb 17 is in an on condition. However,when the temperature of the magnetic material of transformer 15 exceedsthe critical point, the permeability, the coupling and secondary voltageare low and bulb 17 is in an off condition. It will be appreciated thatthis functionally represents a thermally activated single-pole, doublethrow magnetic switch without moving parts.

Referring to FIG. 7, a use of the invention as a digital magnetictransducer is illustrated. Here, the signal propagating apparatuscomprises a set of magnetic cores 20, 21 22 and 23 produced inaccordance with the invention to have different critical temperaturesspaced at given intervals. Each is wound with primary and secondarycoils. The cores are preferably toroidal in shape and the primary coilsare connected together in series and driven by a pulse driver 24. Itwill be appreciated that magnetic cores 20, 21 and 23 are used as linearpulse transformers with the output voltages in the secondary coils beinglinearly proportional to the permeability of the magnetic material amongother things. Should the coils be driven into saturation, the outputvoltage becomes non-linear and proportional to the magnetization insteadof the permeability whereupon a different and undesired temperaturedependence results.

The secondary windings of cores 20, 21, 22 and 23 are connected to apulse detector 25 which preferably is a threshold detector. Pulsedetector 25 detects pulses from the output of those cores 20, 21, 22 and23 which are below their critical temperatures and does not detectoutput pulses from such cores which are above their criticaltemperatures. In this way, pulse detector 25 determines the range oftemperatures of the magnetic cores. The output of pulse detector 25 maybe stored in a shift register 26 from whence the information can bereceived on command. Thus, the register 26 may convert the paralleloutput of cores 20, 21, 22 and 23 and pulse detector 25 into a seriesoutput and shift same out on request.

Referring to FIG. 8, the driving current through the primaries of cores20, 21, 22 and 23 is a triangular wave in time. Thus, inasmuch as suchcores are used as linear pulse transformers, they act in accordance withknown electrical induction laws as differentiators. The output of thesecondary coils is therefore the time derivative of the drive currentand is a square wave in time. Such a preferred configuration of thedrive eliminates the need for integration or additional pulse shapingcircuitry in pulse detector 25.

As a preferred arrangement, pulse detector 25 in FIG. 7 comprisesdigital comparators--one for each core output--which compare theamplitude of the output square wave from cores 20, 21, 22 and 23 to areference voltage. Preferably, such reference voltage is derived fromthe primary current drive by differentiating it in time. Suchdifferentiation may be performed by an air core transformer or othermeans such as a RC network. In such manner, the ratio of the outputvoltage from cores 20, 21, 22 and 23 to the reference voltage remainsindependent of the slope, frequency or exact shape of the drive current.

A magnetic thermometer produced in accordance with the invention givesaccurate results from below -20° C. to above 700° C. An important aspectof the invention lies in the accuracy obtained and its adaptability tothe digital computer art. Magnetic cores produced in accordance with theinvention have been found to have extremely high reliability andreproductibility. They are shock and radiation insensitive and virtuallyindestructible.

A further embodiment of the invention is an apparatus which incorporatesthe magnetic material, which is produced in accordance with theinvention, in a phase-shifting network. With a frequency varying signalbelow 100 Mhz, the apparatus is constructed of discrete elements with amagnetic structure being closed-flux type inductors or transformers. Theself-inductance or mutual-inductance of such inductors or transformerschanges drastically at the critical temperature and is thereforeadaptable for utilization, in conjunction with resistive or capacitanceelements well known in the art, to change drastically the phase of thepropagating signal. In a preferred arrangement, a phase-sensitivedetector is utilized as a responsive means and the output of the phasedetector drastically and discontinously is altered at the criticaltemperature.

With frequencies above 100 Mhz, the signal propagating apparatus is ofthe waveguide type and the magnetic structure of proper geometry isappropriately located inside the wave propagating guide. Through thismeans, the phase of the propagating signal is again drastically alteredat the critical temperature due to the change in permeability whichtakes place in the magnetic structure. A phase-sensitive responsivecomponent such as a micro-wave balanced bridge is preferably utilized asa responsive means with the output of the bridge being drastically anddiscontinuously altered at the critical temperature.

If desired, the type of varying signal may be a pulse or a series ofpulses of a particular shape such as, for example, a square shape. Whensuch pulse or pulses travel along the magnetic structure, the shape ofthe pulse is changed if the magnetic material is below its criticaltemperature. However, the shape remains unchanged with the magneticmaterial above the critical temperature. The reason for this is that thevelocity of propagation of the pulse through the magnetic material is afunction of its frequency. Thus, a shape detector, preferably acorrelator is utilized as the responsive means and the apparatusoperates as a temperature responsive device having an output whichdiscontinuously alters at the critical temperature.

All temperatures unless otherwise specified, are in centigrade. As usedin the claims, the term "imperfections" is intended to include voids,impurities, and distortions which disrupt the symmetry of the crystalinestructure of the materials involved.

Having described my invention, what I desire to secure by Letters Patentof the United States is:
 1. In a method of producing a composition whichin its crystalline state is magnetic and has a substantially constantpermeability at temperatures up to its transition temperature and anabrupt reduction in permeability at a predetermined transitiontemperature for use in detecting with a high degree of accuracy whetherthe ambient temperature is above or below a predetermined transitiontemperature of the composition in said crystalline state, the process ofproducing a magnetic material which has constant permeability attemperatures within about 0.1 degrees centrigrade of a known transitiontemperature that closely approximates said predetermined transitiontemperature, said process including a step which performs the functionof equalizing the temperature dependency of said material's magneticallyanisotrophy with that of its magnetostatic energy when it is in itscrystalline state.
 2. A method in accordance with claim 1, wherein saidimpurities or imperfections consist of atoms of an element which diplacesome of the atoms of said metal in said resulting crystalline state andhave a large atomic diameter relative to said metal atoms, said secondmetal atoms being introduced into said substance by being uniformlymixed therewith.
 3. A method in accordance with claim 2, wherein saidtransition temperature, obtainable by mixing starting materials reagentquality pure and in weight ratios accurate to one part per million,could be closely approximated to about plus or minus 2° C. of saidtransition temperature and is further controlled to obtain an accuracywithin 0.1° C. of said transition temperature by uniformly diffusingtherein a predetermined amount of a soluble salt of iron, manganese, ornickel which is not so great as to alter the circumstance that thecomposition in its magnetic state has a substantially constantpermeability at temperatures up to its transition temperature and anabrupt reduction in permeability at such transition temperature.
 4. Amethod in accordance with claim 2, wherein said metal is cobalt and saidsecond metal is iron.
 5. A method in accordance with claim 3, whereinsaid metal is iron and said second metal is nickel.
 6. A method inaccordance with claim 3, wherein said metal is manganese and said secondmetal is germanium.
 7. A method of producing a magnetic article having adesired transition temperature within an accuracy of at least 0.1° C.which comprises the steps of:(1) providing a homogeneous mixture in apowdered form which in its crystalline state is magnetic, the mixtureconforming to the formula Fe_(n-x)) Mn_(x) Ge as necessary to provide apredetermined transition temperature range of said homogeneous mixturein its crystalline state which is within 2° C. of said desiredtransition temperature and wherein "n" is 3 or 5 and "x" is less than"n" and greater than 0; (2) introducing imperfections into said mixturehomogeneously in the form of added Mn in sufficient amounts as necessaryto provide that the magnetic permeability of the resulting mixture inits crystalline state remains substantially constant with changes intemperature up to said predetermined transition temperature, the mixturewith said added Mn conforming to the formula Fe.sub.(n-x) Mn.sub.(x+a)Ge where "a" is a number between 0.05 and 0.2; (3) introducing by liquiddiffusion into the resulting mixture from step (2) above iron ions insufficient quantities to provide the mixture resulting therefrom with atransition temperature in its crystalline state which is within at least0.1° C. of said desired transition temperature and has a magneticpermeability which remains substantially constant with changes oftemperature up to its transition temperature, said last resultingmixture conforming to the formula Fe.sub.(n-x+b) Mn.sub.(x+a) Ge where"b" is a number in the range of 10⁻¹ to 10⁻⁶ ; (4) forming the lastresulting mixture into the desired shape of said article and preheatingit to a temperature sufficient to decompose all constituents added bythe liquid diffusion step (3) except the iron atoms; and (5) sinteringthe aforesaid shaped article at a temperature less than the meltingpoint of the mixture by not more than 100° C., and thereafter annealingand slowing cooling said article to convert it into an article ofmagnetic crystalline structure having the desired transition temperaturewith accuracy within 0.1° C. or better.
 8. A method of making aspecifically structured ferromagnetic composition which hassubstantially constant permeability at temperatures up to its transitiontemperature and an abrupt reduction in its permeability at apredetermined transition temperature which is employed to detect with ahigh degree of accuracy whether the composition is above or below saidpredetermined transition temperature, the method including the step ofproducing a magnetic material which in its specifically structuredcrystalline state will have a known constant transition temperature thatapproximates within a plus or minus 2° C. said predetermined transitiontemperature, said step including the introduction homogeneously intosaid material mettalic atoms which sufficiently distort said materialwhen in its said crystalline state so that the temperature dependency ofits anistropic energy is the same as that of its magnetostatic energy.9. A method according to claim 1, wherein said step comprisesintroducing homogeneously into a substrate which includes a first metalselected from one of the groups consisting of the iron group, atomicnumbers 22-30, the palladium group, atomic numbers 40-48, the platinumgroup, atomic numbers 72-80, or lithium, atomic number 3, wherein saidsubstance has a crystalline state in which it is magnetic, sufficientimpurities or imperfections comprising a second metal selected from thegroup consisting of zinc, titanium, zirconium, cadmium, yttrium and anyof the aforesaid metals of the iron, palladium, and platinum groups andlithium, so that the temperature dependency of the magnetocrystallineanisotrophy and that of the magnetostatic energy of the resultingmaterial when it is in its crystalline state are equalized.
 10. A methodof making an intermediate material to be used in producing ferromagneticcrystalline compositions capable of detecting within 0.1° C. selecteddesired temperatures by abrupt reductions occurring in the correspondinginitial magnetic permeabilities of such magnetic crystallinecompositions at said selected temperatures, said intermediate materialconsisting essentially of a material which in its crystalline state ismagnetic and has a constant transition temperature that is within plusor minus 2° C. of at least one of said selected desired temperatures,the method comprising a step of performing the function of equalizingthe temperature dependency of said material's anisotropic energy andthat of its magnetostatic energy when in said crystalline state.
 11. Amethod in accordance with claim 10, wherein said step comprisesintroducing homogeneously into said material metallic atoms whichsufficiently distort the crystalline structure of said material when insaid crystalline state so that the temperature dependency of itsanisotropic energy is the same as that of its magnetostatic energy.